The present invention relates to an optical wavelength-division multiplexing transmission system, and, more particularly, to technology for solving problems caused from gain characteristics of an optical amplifier.
Recently, in an optical transmission system, use of an optical amplifier/repeater, and wavelength-division multiplexing of optical signals have been investigated to meet demand for longer distance transmission in higher capacity.
In a conventional long distance wavelength-division multiplexing optical transmission system, a number of direct optical amplifiers are disposed between an optical transmitter and an optical receiver. However, the direct optical amplifier exhibits slightly different gain characteristics for each wavelength used. Accordingly, when a number of direct optical amplifiers are disposed, the gain characteristics of each direct optical amplifier are accumulated. Thus, there arises significant difference between the gain characteristics for each wavelength, and the characteristic curve of the direct optical amplifier becomes sharper, so that the pass band (for example, 3 dB) is narrowed down, and optical signals are significantly attenuated outside the bandwidth.
Japanese Patent Application Laid-Open No. 5-37472 discloses a configuration of an optical transmission system which solves this problem. With such a configuration, first, multiplexed optical signals output from an optical amplifier are divided into each wavelength by an optical divider. Then, an optical attenuator attenuates light in each wavelength so that optical signals with higher gain are matched to those with lower gain. The attenuated light is again multiplexed and output by an optical coupler 7.
However, with the above configuration, since the attenuator accommodating to specific wavelength is provided, the wavelength is fixed, and the arrangement of device is complicated. Moreover, since optical signals with other wavelengths are attenuated to match those with lowest gain, there arises a problem that loss is increased as a whole. Therefore, the configuration of FIG. 4 is difficult to be applied to an optical transmission system over a long distance.